202 PRESERVATION OF WOOD. 
of wood to be impregnated are placed perpendicularly in the receptacle, with 
the larger end downwards, that the solution may ascend in the direction fol- 
lowed by the natural sap; a cover furnished with some openings, and well 
secured, is then applied, and first the solution and afterwards the steam ad- 
mitted; the heating of the solution will thus be effected within two hours. It 
is to be observed that, as the volume of the solution is increased about one-fifth 
by the condensation of the admitted vapor, a proportional quantity of the metallic 
salt should be added to each receptacle to restore the reduced strength. Hach 
of the receptacles is calculated to receive 40 railroad sills, of which 160 may 
therefore be prepared by such an apparatus as that above described within 
twenty-four hours. 
Tt. 
The chemical processes which take place in the impregnation of wood with 
the copper vitriol have been explained by Kénig, of Dresden, through experi- 
ments made chiefly with pine wood. As regards the question whether the wood 
forms a chemical combination with the vitriol or one of its ingredients, he found 
that the oxide of copper, as well as the sulphate, is taken up by the wood, and 
that after washing the wood with water a saline base remains behind. If wood 
thus impregnated be closely observed, it is seen, from the green tincture of par- 
ticular spots, that the metallic salt is deposited between the yearly rings of the 
wood in the less solid parts, and hence in those chiefly filled with sap. It has 
been further observed that wood abounding in resin takes up much more of the 
copper-salt than that which is deficient in it—oak wood, for instance, being 
scarcely stained by the solution. The woody fibre would seem, therefore, to 
have little or nothing to do with fixing the saline principle; it has been shown, 
indeed, that pure fibre—chemically prepared cotton, for example—does not 
combine with it in the slightest degree, but yields it up entirely through repeated 
washings. If, by treatment with alcohol, we obtain wood wholly free from 
resinous constituents and attempt to impregnate it, no color is communicated as 
in the case of resinous woods, and, by slight washing, the salt is removed. By 
evaporation of the alcoholic solution we obtain, under the form of a resinate, a 
greenish residuum containing resin and oxide of copper. It results from these 
interesting observations that the elements of the copper-vitriol are fixed in the 
wood through the medium of its resin. 
If, with a view to a satisfactory determination of the question whether other 
ingredients of the wood may not co-operate in the fixation of the metallic salt, 
we examine the same wood before and after impregnation, it will be found that 
the impregnated wood contains less nitrogen, and that it is even possible, 
through continued treatment of the wood with the vitriolic solution, wholly to 
extract its nitrogenous constituents; these will be discovered in the solution. 
In this we find an explanation of the fact that impregnated wood resists decay 
longer than wood not thus prepared. 
The preservation of wood by means of copper-vitriol depends, under all cir- 
cumstances, upon the condition that the compound resulting from the union 
of the copper and resin should more or less completely fill the pores of the 
wood and invest the woody fibre, thus preventing the access of oxygen, and at 
the same time repelling the attacks of insects. These facts agree with the 
results realized in practice. It has been found that soft wood of loose structure 
lasts after impregnation longer than more solid wood, in conformity with the 
before-cited experiments, which show that the nitrogenous constituents are more 
ey discharged by the copper-vitriol from soft than from hard and heavy 
wood. 
: The experiments of Kénig furnish, however, the mode in which the vitriolic 
lmpregnation may be most advantageously effected. With thin wood it is 
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